ORIGINAL: SeamusG
Here's a typical Helical Coil Insert kit.
. Drill the hole - alignment might well be the biggest challenge
. Tap the hole
. Screw in the insert
. Break off tab
The glow plug is (rumored) to be a 1/4-32.
Good luck.
What was the price of the kit and where did you get it. I have the 1/4 32 tAP TO CLEAN UP THE THREADS but I have never stripped them out but it may be something to have in the shop for that just in case.